schema: allow to pass is_multikey func opt without sub-table

Currently, `is_multikey` function option must be wrapped in `opts`
sub-table, which looks ugly:

```lua
box.schema.func.create('my_func', {
    is_deterministic = true,
    opts = {is_multikey = true}
})
```

Let's allow to pass the option directly.

Note, the `is_multikey` option is reported by `box.func` at the same
level as `is_deterministic` so this would only be consistent:

```lua
box.func.my_func.is_deterministic
box.func.my_func.is_multikey
```

@TarantoolBot document
Title: Document that is_multikey func opt may be passed without sub-table

It's not necessary anymore to wrap `is_multikey` in `opts` table when
creating a function:

```lua
box.schema.func.create('my_func', {is_multikey = true})
```

is equivalent to

```lua
box.schema.func.create('my_func', {opts = {is_multikey = true}})
```

which still works, but deprecated.

Pease update [this][func-index] documentation entry.

[func-index]: https://www.tarantool.io/en/doc/latest/reference/reference_lua/box_space/create_index/#creating-a-functional-index